Photothermographic sheet constructions usually comprise a light-insensitive silver source (e.g., an organic silver salt such as silver behenate or silver benzimidazole, or a complexed silver salt) in catalytic proximity to photographic silver halide. A thermographic developing agent, that is a reducing agent for silver ion, is also present in the construction. Light imaging generates a latent image on the silver halide material. This latent image is essentially the same as a latent image on conventional photographic processes and comprises stable groups of silver metal atoms formed on the silver halide by the action of the imaging light. It is well known that the presence of silver metal is autocatalytic to the reduction of silver ions in electronic proximity (near enough to allow electron transfer) to the silver metal. The formation of the latent image on the silver halide thus generates catalytic silver sites which are in catalytic proximity to the thermographically developable silver source and will catalyze the thermographic reaction. Upon heating the sheet with these catalytic silver sites present therein, thermographic development (reduction of the silver source by the reducing agent for silver ions under the impetus of heat) occurs very rapidly about the catalytic sites and very slowly, if at all, where the silver source is not catalytically activated. A negative image is thus produced, with optical densities generated in light struck areas.
A significant problem with this technology has been the fact that the silver halide, silver source, and reducing agent in non-light struck areas still remain active in the sheet after thermographic development. After prolonged exposure of the final photothermographic image to light, the silver halide will have been converted almost completely to silver metal and catalytic sites will be present over the entire sheet. Even at room temperature, the presence of the catalytic sites will be sufficient to cause the non-imaged background areas to progressively generate a spurious image. This spurious image is formed by the slow activity of the reducing agent acting at ambient temperatures on the catalyzed silver source. Theoretically, the entire sheet would eventually darken completely, destroying the original image. In fact, photothermographic sheets do show intolerable rises in Dmins and general discoloring after years of prolonged and intense exposure to radiation.
Previous attempts have been made to stabilize the final photothermographic silver image, but these attempts have met with mixed success. U.S. Pat. No. 3,707,377, for example, disclosed the use of halogen containing organic compounds as dry silver stabilizers. A test was presented for distinguishing useful halogen containing compounds from useless materials. This test characterization proved to be wrong, with significant numbers of useful halogen containing organic compounds excluded by the test and numerous otiose materials included by the test definition. Although the actual compounds tested in that patent performed properly according to that test, extrapolation beyond those compounds proved to be an error. The degree of stabilization provided by those compounds was also not as great as desired.
U.S. Pat. No. 3,874,946 shows the use of a tribromomethane sulfonylbenzthiazole as a stabilizer for dry silver sheet. This compound generally works well as a stabilizer in photothermographic constructions, the benzthiazole radical being well known as a photographic stabilizer.